Amniotic fluid is clear liquid that cushions and protects the fetus during development. It also provides the fetus with fluids and promotes normal development of the lungs, gastrointestinal tract, muscles and bone. The amount of fluid is controlled by both the fetus and placenta. 

During pregnancy, a woman will have her amniotic fluid measured. Obstetricians-gynecologists (ObGyns) use ultrasound to measure the depth of the fluid in four areas of the uterus. The measurements are added up to determine the amniotic fluid index. These measurements may reveal a number of complications, including:

• Oligohydramnios. This is a condition in which there is too little amniotic fluid. A woman may be diagnosed with this condition when her amniotic fluid depth is less than 5 centimeters (cm). This can occur when there are certain problems with the fetus (e.g., lack of kidneys, abnormal kidneys) or if there are abnormalities in the placenta. According to the March of Dimes Birth Defects Foundation, this condition affects about 8 percent of pregnancies. When it occurs in the first half of pregnancy, the condition is linked to birth defects of the lungs and limbs, as well as an increased risk of miscarriage, premature birth and stillbirth. When the condition occurs later in the pregnancy it can cause poor fetal growth and complications during labor and delivery. In some cases, the fluid may be replaced with an artificial substitute once the patient is in labor (amnio infusion).
  

  

• Polyhydramnios. This is a condition in which there is too much amniotic fluid. A woman may be diagnosed with this condition when her amniotic fluid depth measures greater than 25 cm. This can happen with fetal abnormalities (e.g., problems with swallowing) and with certain abnormalities in the placenta. The March of Dimes estimates that 2 percent of pregnant women develop polyhydramnios. It may increase the risk of premature birth, preterm rupture of the membranes, umbilical cord accidents, placental abruption, poor growth and stillbirth. Women with the condition are also more likely to have a Caesarean section (C-section) and to experience severe bleeding after delivery. Polyhydramnios may be treated by removal of excess fluid or drugs that limit fluid levels.

The placenta is an organ that forms from the same cells as the fetus. It attaches to the wall of the uterus, and the umbilical cord develops on the side of the placenta nearest the fetus. During pregnancy, the placenta forms connections with the mother’s blood supply to provide oxygen and nutrients to the fetus. It also connects to the fetus’s blood supply in order to remove waste products. In addition, the placenta protects the fetus from infections and harmful substances.

Placental problems are among the most frequently occurring complications in the second half of pregnancy. Common placental problems include:

• Placental abruption. Also known as abruptio placentae, placental abruption is a serious complication in which the placenta partially or completely separates from the wall of the uterus before delivery. As a result of this detachment, the fetus can be deprived of oxygen and nutrients and the mother can suffer life-threatening bleeding. According to the March of Dimes, placental abruption occurs in approximately 1 in every 100 pregnancies. However, the risk is increased for women who smoke cigarettes, use cocaine or have high blood pressure during pregnancy. Although it occurs most often in the third trimester, the condition can develop any time after the 20^th week of pregnancy. The main sign of placental abruption is bleeding. Ultrasound can be used to diagnose the condition.
  
  
• Placenta accreta, placenta increta, placenta percreta. In some pregnancies, the placenta may attach itself too deeply and too firmly into the uterine wall (placenta accreta), attach itself even more deeply into the uterine wall (placenta increta) or attach itself through the uterus, sometimes extending to nearby organs (placenta percreta). According to the March of Dimes, these disorders occur in approximately one in every 2,500 pregnancies. They generally do not cause symptoms until the third trimester and may result in vaginal bleeding and preterm delivery. Because these conditions prevent the placenta from easily detaching from the uterine wall after delivery, they often require surgical removal of the placenta, and in some cases, a hysterectomy. These conditions also occur more frequently in women who have had a previous C-section.
  
  
• Placenta previa. A condition in which the placenta lies low in the uterus, partially or completely covering the internal opening of the cervix. The March of Dimes estimates that the condition occurs in approximately one in every 200 pregnancies. It is more common in women who smoke, use cocaine, or are over the age of 35 years. Placenta previa may cause painless vaginal bleeding during the second half of the pregnancy. The condition may be detected during a routine ultrasound examination. When the condition is diagnosed during the second trimester, it usually corrects itself by the end of the pregnancy. If the placenta previa has not resolved by the latter part of the pregnancy, the expectant mother may require hospitalization and monitoring until delivery. Because there is a risk of severe bleeding, women with placenta previa may undergo a C-section.